X-ray tomographic imaging, in its simplest expression, is an X-ray beam traversing an object, and a detector relating the overall attenuation per ray. The attenuation is derived from a comparison of the same ray with and without the presence of the object. From this conceptual definition, several steps are required to properly construct an image. For instance, the finite size of the X-ray generator, the nature and shape of the filter blocking the very low-energy X-rays from the generator, the details of the geometry and characteristics of the detector, and the capacity of the acquisition system are all elements that affect how reconstruction is performed.
Conventional X-ray detectors integrate the total electrical current produced in a radiation sensor, and disregard the amplitude information from individual photon detection events. Since the charge amplitude from each event is proportional to the photon's detected energy, this acquisition provides no information about the energy of individual photons, and is thus unable to capture the energy dependence of the attenuation coefficient in the object.
On the other hand, semiconductor X-ray detectors that are capable of single photon counting and individual pulse-height analysis may be used. These X-ray detectors are made possible by the availability of fast semiconductor radiation sensor materials with room temperature operation and good energy resolution, combined with application-specific integrated circuits (ASICs) suitable for multi-pixel parallel readout and fast counting.
One major advantage of such photon-counting detectors is that, when combined with pulse-height analysis readout, spectral information can be obtained about the attenuation coefficient in the object. A conventional CT measures the attenuation at one average energy only, while in reality, the attenuation coefficient strongly depends on the photon energy. In contrast, with pulse-height analysis, a system is able to categorize the incident X-ray photons into several energy bins based on their detected energy. This spectral information can effectively improve material discrimination and target contrast, all of which can be traded for a dose reduction to a patient.
Many clinical applications benefit from spectral CT capabilities such as material decomposition and beam hardening direction. One of the technical difficulties with a photon-counting detector system for general purpose CT is the limited count rate ability of the detector. Furthermore, semiconductor-based detectors have the problem of inter-pixel crosstalk. High X-ray flux, commonly encountered in CT scans, causes CdTe/CdZnTe-based photon-counting detectors to polarize and stop functioning. Thus, it is very important to achieve desirable detector responses under high flux. Furthermore, scattering correction is a very big technical challenge for multi-slice sparse fourth generation PCCT geometry.